Sapphires or corundums are formed of aluminium oxide crystals (Al2O3) whose colour is determined by the presence of trace impurities (oxides) in these materials. It is thus known that the presence of titanium and iron gives a sapphire a blue colour, the presence of vanadium a violet colour and the presence of chromium a pink colour. Finally, the presence of iron in a sapphire gives the latter a yellow or green appearance.
This colouration can be explained by the fact that the impurities cause the appearance of energy levels in the forbidden gap of corundum, which modify the transmission and absorption spectra of the material and therefore its colour.
Sapphire can be heat treated; stones that are too light or too dark or have too many inclusions are heated. This heat process enhances the colour and improves the brilliance of the stones by dissolving the trace impurities present in the stone.
The laboratory manufacture of synthetic sapphires and rubies has been known since the beginning of the 19th century. The chemical composition and physical properties of these synthetic stones are the same as those of natural stones. Synthetic stones can, however, be distinguished from natural stones by their generally curved crystallisation lines, at least as regards the oldest manufactured synthetic stones.
Because of its high scratch-resistance, synthetic sapphire is used, in particular, for making watch crystals or lenses in optical cameras, especially in smart phones. Nowadays, the production of synthetic sapphires is carried out on an industrial scale.
It is well known that a synthetic sapphire surface reflects around 15% of incident light, which hinders the reading of information displayed by a watch dial, a flat computer or mobile telephone screen.
The light reflectivity value of a synthetic sapphire surface is obtained using Fresnel's equations which, for a light ray passing through a dioptre at an angle of incidence of 90°, give the following reflection (R) and transmission (T) coefficients:R=((n2−n1)/(n2+n1))2 T=4n1*n2/(n2+n1)2 where n1 and n2 are the reflection indices of the mediums separated by the dioptre.
Taking account of the principle of energy conservation, one obtains R+T=1.
For air (n1=1) and synthetic sapphire (n2=1.76), one obtains, with the above formulae: R=0.0758, T=1−R=0.9242. In other words, 7.6% of the visible light that falls perpendicularly on a synthetic sapphire surface is reflected, and 92.4% of this light is transmitted.
For a strip of synthetic sapphire formed of an entry face and an exit face which extend parallel to and remote from each other, the optical losses are two times higher and are thus around 15%. This high reflection of ambient light makes it difficult to read the information displayed, for example, by a watch dial (hands, date indication, decorations) located under the synthetic sapphire watch crystal.
There are anti-reflective methods consisting in depositing metal oxides which are relatively complex and expensive to implement. For example, for watch crystals, one of the methods used consists of the vacuum deposition (10−5 torr) of metal oxide thin films. In dust-free enclosures of the white room type, the watch crystals are first cleaned in cleaning lines and then ultrasonic dried. These watch crystals are then mounted on supports which are inserted into vacuum chamber Bell jars. The vacuum inside these vacuum chamber Bell jars allows the evaporation by sublimation of a metal oxide at a lower temperature than under atmospheric pressure. Evaporation can be achieved by Joule heating the metal oxide or by bombarding the oxide using an electron gun. The quality of the vacuum, the speed of evaporation and the thickness of the deposited films must be perfectly controlled. These thicknesses must of course be uniform.
There are other, less expensive, types of vapour phase depositions (also known as physical vapour deposition or PVD) which consist in depositing magnesium fluoride MgF2 (optical refractive index 1.38) or cryolite Na3A1F6 (optical refractive index 1.35). The refractive indices of these materials are close to one another, but their scratch-resistance properties are inferior to those of synthetic sapphire. PVD depositions made on synthetic sapphire to improve its anti-reflective properties can be scratched, or peel off, thereby completely removing any advantage that could be obtained for synthetic sapphire.
‘Synthetic sapphire’ means a material transparent to visible light. Synthetic sapphire is formed of aluminium oxide (Al2O3). In physical terms, synthetic sapphire is a very hard crystalline material (hardness equal to 9 on the Mohs scale) belonging to the corundum family and which has a very high refractive index equal to 1.76.
More generally, the present invention concerns any type of material such as, but not limited to, synthetic sapphire, polycarbonate, mineral glass or ceramics. The treated materials may be electrically conductive, or semiconductors, or electrically insulating.
Other known surface treatment techniques consist of implanting ions in the surface of a treated object.
These ion implantation methods consist of the surface bombardment of the treated object, for example by means of a source of single or multiply charged ions of the electronic cyclotron resonance type. This type of device is also known as an ECR ion source.
An ECR ion source makes use of electron cyclotron resonance to create a plasma. Microwaves are injected into a volume of low pressure gas intended to be ionised, at a frequency corresponding to the electronic cyclotron resonance defined by a magnetic field applied to an area located inside the volume of gas to be ionised. The microwaves heat the free electrons present in the volume of gas to be ionised. By thermal agitation, these free electrons will collide with the atoms or molecules and cause their ionisation. The ions produced correspond to the type of gas used. This gas may be pure or compound. It may also be a vapour obtained from a solid or liquid material. The ECR ion source is capable of producing single charged ions, i.e. ions whose degree of ionisation is equal to 1, or multiply charged ions, i.e. ions whose degree of ionisation is higher than 1.
Within the scope of the present Patent Application, we are concerned with a single or multiply charged ion source of the ECR type. Very briefly, and as illustrated in FIG. 1 annexed to the present Patent Application, an ECR ion source, designated as a whole by the general reference numeral 1, includes an injection stage 2 into which is introduced a volume 4 of gas to be ionised and a hyperfrequency wave 6, a magnetic confinement stage 8 in which a plasma 10 is created, and an extraction stage 11, which allows the ions from plasma 10 to be extracted and accelerated by means of an anode 11a and a cathode 11b between which a high voltage is applied.
The aspect of ion beam 12 produced at the exit of multiply charged ECR ion source 1 is illustrated in FIG. 2 annexed to the present Patent Application. It is noted that this ion beam 12 tends to diverge at the exit of ECR ion source 1, which can be explained by the fact that the ions, which all have the same electrical sign, tend to repel each other. Since ion beam 12 tends to diverge at the exit of ECR ion source 1, this causes problems of inhomogeneity of ion distribution at the surface of the treated object.
Another problem linked to surface ion implantation of a treated object concerns the gradual appearance of electrostatic potential at the surface of the treated object as the single or multiply charged ions are deposited. Indeed, the higher the number of ions implanted in the surface of the treated object, the higher the electrostatic field, and the more the surface of the treated object tends to repel the ions arriving from the ECR ion source, which also causes problems of inhomogeneity in the method for ion implantation of the treated object. In the case where the treated object is electrically conductive, this problem is less present insofar as at least some of the free or weakly bound electrons of the material from which the treated object is made can recombine with the implanted ions. However, in the case where the treated object is made of a non-electrically conductive material, the phenomenon of recombination between electrons and single or multiply charged ions does not occur, and it is difficult to guarantee homogeneous ion distribution at the surface of the treated object.